This invention relates to compact heat exchangers of the type generally known as plate-fin heat exchangers, and in particular to the structure and manufacture of high-integrity heat exchanger elements for such heat exchangers.
General information on plate fin heat exchangers may be found, for example, in a 1983 publication entitled "Compact Heat Exchangers" by R.L. Webb, published by the Hemisphere Publishing Corporation and obtainable from the British Library at Boston Spa, Yorkshire, England. A more comprehensive guide is "Plate-Fin Heat Exchangers - A Guide to their Specification and Use", 1st. Edition, edited by M.A. Taylor and published in 1987 by the Heat Transfer And Fluid Flow Service (HTFS) of Harwell Laboratory, Harwell, England.
The core layer of a plate-fin heat exchanger element is the space between adjacent plates of the heat exchanger matrix through which the heat exchange fluid flows. The core layer contains the so-called fin elements which aid the heat exchange process as the heat exchange fluid flows past them. In some heat exchange configurations the fin elements effectively comprise partitions running longitudinally of the core layer to define discrete fluid flow passages extending between the inlet and outlet regions. In others they are discontinuous elements around which the heat exchange fluid flows between the inlet and outlet regions.
Plate-fin heat exchanger elements require an effective structure for distributing the flow of heat exchange fluid from the fluid inlet of the element evenly across the width of the core layer. Also required is structure for collecting the fluid from across the width of the core layer and concentrating it towards the outlet of the element. Both types of structure are similar and will be referred to as distributors and collectors in the following description and claims.
One known way of providing plate-fin heat exchanger elements with distributors and collectors is to braze corrugated sheets into the inlet and outlet regions of the elements, the corrugations being arranged to convey the flow from the inlet to the main heat exchange region comprising the fins and from the fins to the outlet This is consonant with the prevailing practice in manufacture of plate-fin heat exchangers, which relies on assembling a multitude of parts into a multi-layer sandwich and brazing them together to produce a complete heat exchanger matrix.
The present invention seeks to provide elements for heat exchangers which have improved integrity, reduced complexity and reduced cost of fabrication in comparison with conventionally produced elements of the plate-fin type.
According to one aspect of the present invention, a heat exchanger element comprises
a core layer for flow of heat exchange fluid therethrough, and PA0 fluid inlet and outlet means communicating with the core layer; PA0 a main heat exchange region containing heat exchange surfaces, and PA0 a distributor region containing flow intercepting surfaces, the distributor region extending between the main heat exchange region and the inlet means so as to distribute the fluid flow across the extent of the core layer;
the core layer including
the heat exchange element comprising at least two unitary sheets of metal joined by diffusion bonding in selected places, at least one unitary sheet having been superplastically deformed, in at least some of the places where it has not been diffusion bonded, so as to form the core layer and define therein the flow intercepting surfaces in the distributor region as well as the heat exchange surfaces in the main heat exchange region.
The flow intercepting surfaces in the distributor region may comprise corrugations in the at least one superplastically deformed sheet, the corrugations being arranged to convey the heat exchange fluid from the inlet means to the main heat exchange region; in this case the minimum number of sheets required to make a single heat exchanger element will be two.
If necessary, the at least one superplastically deformed sheet may have a relatively undeformed transitional portion between a part of itself which is deformed so as to define the heat exchange surfaces in the main heat exchange region and another part of itself which is deformed so as to define the flow intercepting surfaces in the distributor region.
Alternatively, and using a minimum of two sheets of metal joined by diffusion bonding in selected places, we have found that an effective way of distributing the fluid flow across the lateral extent of the core layer is to incorporate flow intercepting surfaces in the distributor region which comprise an array of superplastically formed hollow projections projecting outwardly from a side of the at least one superplastically deformed sheet, the ends of the projections being diffusion bonded to the adjacent sheet.
Preferably, however, the heat exchanger element comprises at least three unitary sheets of metal joined by diffusion bonding in selected places, the flow intercepting surfaces in the distributor region comprising first and second sets of superplastically formed hollow projections, the first set of hollow projections projecting outwardly from a first side of the at least one superplastically deformed sheet, and the second set of hollow projections projecting outwardly from a second side of the at least one superplastically deformed sheet, the ends of the projections being diffusion bonded to the two adjacent sheets.
We also prefer that the core layer additionally comprises a fluid collector region formed integrally with the main heat exchange region and the distributor region and containing flow intercepting surfaces, the collector region extending between the main heat exchange region and the outlet means so as to collect the fluid flow from across the extent of the core layer and concentrate it for flow through the fluid outlet means.
The invention also embraces heat exchanger matrices comprising heat exchanger elements as described above.
According to a further aspect of the present invention a manufacturing method for a heat exchanger element comprises the steps of:
(a) selecting at least two metal sheets for stacking together, at least one of the metal sheets being capable of being superplastically deformed to form an expanded core layer in the finished element for flow of heat exchange medium therethrough;
(b) applying anti-diffusion-bonding stop-off substance in selected interfacial areas of at least one of the sheets thereby to define where diffusion bonding of the sheets will not occur, said interfacial areas being selected to define heat exchange surfaces in a main heat exchange region of the core layer, fluid inlet and outlet means for the core layer, and flow intercepting surfaces in a flow distributor region between the main heat exchange region and the fluid inlet means;
(c) stacking the sheets together and applying heat and pressure across the stack thickness to diffusion bond the sheets together where there is no anti-diffusionbonding substance;
(d) cooling the resulting diffusion-bonded structure;
(e) pressurising the interior of the structure by injecting a suitable fluid to break adhesive bonds between the anti-diffusion-bonding substance and the sheets; and
(f) heating the structure and pressurising its interior to superplastically deform said at least one metal sheet thereby simultaneously producing the heat exchange surfaces in the main heat exchange region and the flow intercepting surfaces in the flow distributor region.
Preferably, between steps (e) and (f) of the above method are inserted the consecutive additional steps of
purging the resulting structure of contaminants using an inert fluid to do so; and
evacuating the structure.
We prefer heat exchange elements according to the invention to have a flow collector region, extending between the main heat exchange region and the outlet means, in addition to a flow distributor region. In such a case, the anti-diffusion-bonding stop-off substance must be applied in step (b) of the above manufacturing method so as to define flow intercepting surfaces in the required flow collector region , whereby during the heating and pressurising step (f) the flow intercepting surfaces in the flow collector region are produced simultaneously with the heat exchange surfaces in the main heat exchange region and the flow intercepting surfaces in the flow distributor region.